Modern fibre reinforced composites, e.g. wind turbine blades, are typically manufactured using Vacuum Assisted Resin Transfer Moulding, a process that produces strong, lightweight composites by infusing resin into compacted reinforcing materials under vacuum. A large part of the reinforcing material is usually glass or carbon fibre woven mats.
For deeply concave shapes of the inner face of the mould there may be a risk that the fibre mats are not maintained in a position firmly against the mould during lay-up.
Under certain circumstances, the fibre mats may tend to take on the shape of catenaries (like a hanging chain), leaving voids between the inner surface of the mould and the fibre mats (“hovering glass”) instead of following the actual curvature of the mould. If several layers of fibre mats are placed on top of each other, friction between the layers may be strong enough to prevent the fibre mats from being pressed against the mould when vacuum is applied.
In the subsequent moulding process the voids between the surface of the mould and the fibre mats will be filled with resin that is not reinforced by any fibre material. As a result, the structural characteristics of the composite in the regions of “hovering glass” may not be as desired.
In addition, if on application of vacuum the glass is pressed partly or completely out into the void this may result in wrinkles and folds of the fibre mats, which may in turn lead to mechanical weaknesses if the wrinkles and folds are not flattened before the resin is injected.
In EP 1 310 351 B1, a manufacturing process of a wind turbine blade is disclosed, wherein a lower part of a mould is filled with layers of fibre glass and core material like balsa wood. Mould cores are covered by vacuum bags and placed in the mould together with a shear web. Then more fibre glass and core material is placed over the mould cores, and an upper part of the mould is put into place. Vacuum is introduced to the region between the vacuum bags and the mould, and resin is injected under vacuum.
EP 2 123 431 B1 describes another method of manufacturing a wind turbine blade, wherein an upper and a lower part of a mould are assembled. The method includes placing a vacuum distributing layer on the inner surface of both, the upper and the lower part of the mould. Each of the vacuum distributing layers is connected to a vacuum pump for applying suction. A layer of fibre glass mats is placed on the inner surface of the vacuum distributing layer and one or more layers of fibre glass mats are added, together with a layer of core material like balsa wood.
During the lay-up of the different layers, suction is applied between the inner surface of the mould parts and the layers by means of the vacuum distributing layer.
After having completed the lay-up in both parts of the mould, the mould cores with the vacuum bags and the shear web are placed in the lower part of the mould. Next, the upper part of the mould is turned 180 degree around its longitudinal axis and is put in place such that the mould is closed.
All the layers of fibre material in both parts of the mould, and particularly those that are in contact with the shear web, are suitable for lamination. Thus, the shear web becomes firmly integrated in the laminated blade structure.
Close to the edges of fibre glass lay-up the pressure between the individual layers is higher than elsewhere. Therefore suction may not be sufficient to prevent the layers from peeling away from the mould at the edges. To compensate for this, EP 2 123 431 B1 suggests to place a layer of a material which has a lower air permeability than the fibre mats in the mould on top of the outermost of these fibre mats. Placing such a layer, which may be referred to as a surface fibre material layer, on top of the outermost fibre mat increases the suction that holds the one or more fibre mats in place.
A number of conflicting requirements exist in relation to the above-mentioned surface fibre material layer.
On the one hand, it is desired that the surface fibre material layer has low permeability to air. In tendency, this leads to a type of fibre mat that is special, has relatively poor resin wetting properties and lower interlaminar shear strength than what is normally desirable.
On the other hand, it is desired that the surface fibre material layer, which forms part of the structural reinforcement of the laminate, is of a relatively conventional type, i.e. has good resin wetting properties and a high interlaminar shear strength relative to any joining structural elements, such as shells, beams, etc. In tendency, these requirements all lead to a fairly open, relatively permeable type of fibre material layer.
Furthermore, even with a relatively low air permeability of the outermost fibre layer, an extremely large air flow is necessary to maintain a sufficient pressure difference over the fibre lay-up, especially when moulding large composites like wind turbine blades.